Protecting generators in transmission plants.



Patented July 9, l90l.

T. J. JOHNSTON. PROTECTING GENERATORS IN, TRANSMISSION PLANTS.

{Application fiIgihl-pb. 18, 1899.)

(M Model.)

Inventor,

Witnesses.

ThomasJJohn st on by qtby.

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THOMAS J. JOHNSTON, OF SOHENEOTADY, NEiV YORK, ASSIGNOR TO THE GENERAL ELECTRIC COMPANY, OF NEIV YORK.

PROTECTING GENERATORS IN TRANSitllSSiON PLANTS.

SPECIFICATION forming part of Letters Patent No. 678,185, dated July 9, 1901.

Application filed February 18, 1899. Serial No. 705,961. (No model.)

To ctZZ whom it may concern.-

Beit known that I, THOMAS J. JOHNSTON, a citizen of the United States, residing at Schenectady, in the county of Schenectady, State of New York, have invented certain new and useful Improvements in Protecting Generators in Transmission Plants, (Case No. 1,110,) of which the following is a specification.

The present invention relates to the pro- IO tection of generators, particularly of alter-.

nating-current transmission plants. It is in some respects applicable to any separatelyeXcited machine; but its greatest utility will be found in the case named. Great difficulty I5 arises in breaking the circuit in such plants because of the high voltage on the transmission-lines. Where the main circuit is itself broken, it is not unusual for the arc to hold across from one side to the other of the circuit-breaker, sometimes even destroying the devices which are intended to open the circuit or prevent the are from spanning the contacts. I therefore provide a circuit-breaking apparatus in the field-magnet circuit of the generator with a current-measuring device operated from the line and acting upon the circuit-breaker, so that when the current rises to a determined maximum the field-magnet circuit of the generator is opened. For

this purpose I prefer current-transformers in the main line, the tripping-coil of the circuit breaker being in series with the transformer and having in circuit with it a regulable re sistance by which the effect of the coil may be determined. Where the transmission-line supplies rotary converters,'a difficulty arises which prevents ordinary circuit-breaking apparatus being efficacious-that is, when the circuit is opened the potential drops, but al 40 most instantly the rotaries begin to feed back into the line a leading current which furnishes an excitation to the generator-armature, and the latter then feeds the line with current at a potential sufficient in many cases to span the gaps at the circuit-breaker, though not of so high a potential as that for which the plant is primarily designed. To provide against this, I not only open the field-circuit of the generator, so that there can be no inductive effect from the field, but I also provide an arrangement for opening the main circuit, operated by the same current-transformer before referred to. WVhere step-up transformers are used, I prefer to interpose the circuit-breaking arrangement, as Well as the current-transformer, in the primary line of the transformer; but where a high-potenti'al generator is employed this of course becomes impossible aud the apparatus Would be operated directly from the transmissionline. Of course for the purposes of this in- Vention either may be called with propriety the main line.

' In the accompanying drawings, Figure 1 is a diagram showing an embodiment of the in- Vention in the form in which it is used with a step-up transformer. Fig. 2 is an illustrative diagram.

A is the armature, indicated diagrammatically by the three collector-rings of a threephase generator, supplying energy to the circuit, the leads of which are numbered 1 2 3.

A step-up transformer T furnishes the required high potential to the transmission-line a, ll 0. A series or current transformer T is employed to furnish the desired energy for the circuit-breakers O D, the first of which is in the circuit of the field AF of the alternator (fed by the exciter E) and the second of which is in the low-potential side of the stepup transformer. The transformer T has coils t t in two of the lines of the three-phase system, acting upon a common secondary coil i In circuit with this secondary is the trippingcoil 0 of the circuitbreaker O. A regulable resistance R0 in series with it adjusts its effect to the value desired. Of course the resistance might be in multiple with the coil.

A similar coil d acts upon the latch of the circuit-breaker D, and a similar resistance 0 RD adjusts its effect.

The operation of the apparatus is as follows: Assume, for instance, that a short circuit or other undue flow of current occurs between the lines a b. This would be reflected back through the transformer T to the coil t of the transformer T, and a current above that at which the circuit-breakers are set to act would flow in the coils o d. The circuit-. breaker O would open, killing the field AF of I00 the alternator. At the same time the circuitbreaker D would also open, opening the eir' cuit between the transformer T and the gencrater-armature.

In the three-phase system, which I have chosen as a convenient one for illustration of my invention, it is necessary to use only two of the coils 15 t in the current or series transformer. This arrangement is an effective safeguard against an overload between any two of the lines. It also enables me to adjust the circuit-breakers to the desired current-flow in any one of the lines. If three coils were used, there would be, as is well known, no effective magnetization of the transformer, while if the connections of one of the coils should be reversed, as is some times proposed, the effect in the transformer secondary would be twice that due to the current in any one of the-lines, according to the well-known principles of alternating currents. The effect of the two coils is therefore a function of the current-flow in either of them. At the same time, as already pointed out, the desired indication of undue current in any of the three lines is furnished.

By referring to Fig. 2, which is a diagram illustrating, by means of the well-known parallelogram of forces, the currents in the lines, the action will be better understood. the currents in the coils t t are indicated by lines bearing corresponding letters, while the third line it is the resultant of the currentflow in the two coils and indicates the effective magnetization of the secondary by the two coils. It will be seen that where the currents are displaced one hundred and twenty degrees, as in a three-phase system, the resultant t is of the same amplitude as either of the currents if '6.

I do not mean by this explanation to limit the invention to three phase systems only, as it is manifest that it may be conveniently employed and will be of utility not only in altemating-current systems of any number of phases, but under some circumstances with contimuons-current machines.

Here

Nhat I claim as new, and desire to secure by Letters Patent of the United States, is-

1. In a polyphase system, the combination of a series transformer having primary coils in more than one of the lines, and a common secondary for the coils, with a circuit-controlling device in the exciting-circuit of the generator, operated by the current in the sec ondary of the series transformer.

2. Ina three-phase system,tl1e combination of a series transformer having primary coils in two ofthe lines, with a common secondary, and a circuit'breaker supplied with cur rent from the secondary and arranged to open the exciting-circuit of the generator when the current reaches a determined value.

3. The combination with a series transformer in an alternating-current system, ofa circuit-breaker in the main line, and another circuit-breaker in the exciter-circuit, the two connected in multiple to the secondary of the transformer.

4. In a polyphase system, the combination of aplurality of transformers connncted with different mains, a secondarycircuit supplied by the several transformers, and a circuitbreaker governing potential of the system, controlled by the secondary circuit.

5. In a polyphase system, the combination of a series transformer having a single secondary and a plurality of primary coils in series in different lines of the system, with a circuit-breaker for opening all of the lines, having its tripping-coil in series with the secondary of the transformer and an adjustable resistance, and a second circuit-breaker in the generator eXciter-circnit, having its tripping-coil connected in series with the same secondary and in multiple with the trippingcoil of the other circuit-breaker, and a second adjustable resistance in series with the coil.

In witness whereof I have hereunto set my hand this 16th day of February, 1899.

THOMAS J. JOHNSTON.

lVitnesses:

'B. HULL, M. E. JAOOBSON. 

